[Jgeneral] why is _5+_4 not the same as -5 + -4 ???

[elmer_fwd]

If I input:
 
    -5 + -4

I get:

_1

But when I input:

    a=. -5
    b=. -4
    a + b

I get:

_9

Any comments for a newbie - ( did APL about 30 years ago for
6 months )

thanks
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